Edit Appointment Duration Output

I know how to create appointments and how to set the timebase.
Problem is this: I need to create 30 min appointment slots that include 30min of extra time for paperwork, after the appointment. I can easily solve this by selecting 60min as the appoinment time, however when the client books a time, it then would show them "60 minutes approx meeting time".
That is no good.
I need to show the client 30 Min approx time in the appointment confirmation dialog, but for every 30min slot that is booked, i need to have an additional 30min of time for myselft afterwards. ie. the next avail appointment after 8am would only be 9am, not 8:30am.
Thanks!